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Autunite

Autunite is a radioactive phosphate mineral with the chemical formula Ca(UO₂)₂(PO₄)₂·10–12H₂O, classified as a hydrated calcium uranyl phosphate that forms tetragonal crystals and is renowned for its bright yellow-green fluorescence under ultraviolet light.^[1] It typically appears as lemon-yellow to greenish-black tabular crystals or foliated masses with a vitreous to pearly luster, a Mohs hardness of 2–2.5, and a specific gravity of 3.05–3.2, while being highly prone to dehydration in air, which can alter it to meta-autunite.^[1] First described in 1852 near Autun, Saône-et-Loire, France—after which it is named—autunite was formally named in 1854 by mineralogists Henry J. Brooke and William H. Miller based on specimens from the type locality at Saint-Symphorien.^[2] As a secondary mineral, it results from the oxidation and hydrothermal alteration of primary uranium-bearing minerals like uraninite, commonly occurring in veins, granite pegmatites, and the oxidized zones of uranium deposits worldwide, including notable localities in France, Portugal, the United States (such as the Daybreak Mine in Washington), Brazil, and Australia.^[1] Associated minerals often include meta-autunite, torbernite, phosphuranylite, and schoepite.^[1] Due to its uranium content, autunite serves as a minor ore of uranium, which is extracted for use in nuclear fission to generate electricity, though its economic significance is limited compared to primary uranium sources.^[2] Historically, it has also found minor applications in ceramics for producing glazes with a red hue tinged yellow, and in photoelectric tubes as a cathode material, but its radioactivity necessitates careful handling in collections and research.^[2] Optically, it is uniaxial negative with refractive indices around ω = 1.575 and ε = 1.572, showing strong pleochroism from colorless to dark yellow.^[1]

Chemical and Physical Properties

Composition

Autunite is a hydrated calcium uranyl phosphate mineral with the chemical formula Ca(UO₂)₂(PO₄)₂·10–12 H₂O.^[3] The uranium content in this composition is approximately 48.27% by weight.^[4] The mineral exhibits variable hydration states, with 10 to 12 water molecules per formula unit, leading to slight variations in the overall composition due to its zeolitic nature.^[4] This water content can decrease through dehydration, a process that occurs irreversibly under ambient conditions, resulting in chemical instability and transformation to meta-autunite forms with reduced hydration (e.g., 2–6 H₂O).^[3]^[4] In its structure, autunite features calcium as the primary cation, uranyl ions (UO₂²⁺), and phosphate groups (PO₄³⁻), which together define it as a secondary uranium mineral formed through the oxidation of primary uranium-bearing phases.^[4] While the pure end-member composition predominates, minor substitutions may occur, such as barium, magnesium, sodium, or ammonium replacing calcium, along with trace amounts of vanadium, lead, or iron; however, these impurities are typically limited and do not alter the essential phosphate character.^[4]

Crystal Structure

Autunite possesses a layered crystal structure typical of the autunite group, consisting of two-dimensional sheets where uranyl cations (UO₂²⁺) in pentagonal bipyramidal coordination share vertices with phosphate tetrahedra (PO₄³⁻) to form anionic layers of composition [UO₂(PO₄)]⁻. These sheets lie parallel to the (001) plane and exhibit a specific topology in which each uranyl polyhedron connects to four phosphate groups, creating a distorted hexagonal arrangement with phosphate tetrahedra bridging adjacent uranyl units. The sheets are interleaved with layers containing Ca²⁺ cations coordinated to oxygen atoms from water molecules and sheet apices, stabilizing the overall framework through electrostatic interactions and hydrogen bonding. The idealized crystal system of autunite is tetragonal, with space group I4/mmm, reflecting the high symmetry of the uranyl phosphate sheets, although natural samples often display lower orthorhombic symmetry (Pnma) due to partial ordering of interlayer water and cations. The unit cell parameters are a ≈ 7.00 Å, c ≈ 20.70 Å, and Z = 2, corresponding to two formula units per cell and accommodating the variable hydration state (10–12 H₂O molecules). This arrangement results in a basal spacing that reflects the thickness of the sheet plus the hydrated interlayer.^[5] Dehydration of autunite involves the progressive loss of interlayer water molecules, leading to meta-autunite without collapse of the uranyl phosphate sheet framework. This transformation contracts the c-axis parameter to approximately 17.0 Å while preserving the tetragonal symmetry, now in space group P4/ncc with a ≈ 7.00 Å and Z = 2; the sheets remain intact, but the interlayer becomes more compact with reduced coordination around Ca²⁺. Such dehydration occurs readily at ambient conditions and is reversible under humid environments, highlighting the structural flexibility of the mineral.^[6]^[7]

Appearance and Optical Properties

Autunite typically exhibits a lemon-yellow to greenish-yellow color, though specimens may appear pale green or, in rarer cases, dark green to greenish-black due to inclusions or dehydration effects.^[1] The mineral is transparent to translucent in well-formed crystals, with diaphaneity decreasing to translucent or even opaque in dense aggregates or crusts where light scattering occurs.^[1]^[8] The crystal habit of autunite is predominantly tabular on the {001} face, forming thin to moderately thick square or rectangular plates with octagonal outlines, often up to 2 cm in size; these commonly occur as subparallel growths, scaly or foliated masses, crusts, or radiating aggregates.^[1]^[8] Autunite displays a vitreous to pearly luster, with the pearly sheen particularly evident on the {001} cleavage surface.^[1] It shows distinct pleochroism in transmitted light, appearing colorless to pale yellow parallel to the X axis and yellow to dark yellow parallel to the Y and Z axes.^[8]^[1] Optically, it is uniaxial negative with refractive indices ω = 1.575 and ε = 1.572, and a birefringence of approximately 0.003.^[1] Under ultraviolet light, autunite fluoresces with a bright green to yellow-green glow in both long-wave and short-wave spectra, a property attributed to the activation by uranyl ions (UO₂²⁺) within its structure.^[1]^[9] The streak of autunite is pale yellow, and in massive or aggregated forms, the mineral's overall translucency can vary, appearing more opaque due to intergrown crystals.^[1]^[8]

Density and Hardness

Autunite is a soft mineral with a Mohs hardness ranging from 2 to 2.5, allowing it to be easily scratched by common materials such as a fingernail or copper penny.^[1] This low hardness contributes to its overall mechanical fragility, making it susceptible to abrasion and deformation during handling.^[8] The specific gravity of autunite is measured between 3.05 and 3.2 g/cm³, with calculated values around 3.14 g/cm³ for compositions containing approximately 10.5 water molecules; these values can vary slightly due to differences in hydration levels.^[1] Autunite displays perfect cleavage on the {001} plane and indistinct cleavage on {100}, enabling it to readily split into thin, tabular sheets along these directions.^[1] Its fracture is micaceous, exhibiting a flaky or layered break, while its tenacity is sectile, meaning it can be cut into thin shavings; however, the mineral remains fragile and may split easily under minimal pressure.^[8] This combination of properties underscores the need for cautious manipulation to avoid inadvertent breakage. Autunite is also soluble in acids, which further highlights its chemical vulnerability alongside its physical softness.^[8]

Geological Occurrence

Formation Processes

Autunite is a secondary uranium mineral that primarily forms through the oxidation of primary uranium-bearing minerals, such as uraninite (UO₂) and pitchblende, followed by hydration in the presence of calcium and phosphate ions.^[10] This process involves the conversion of tetravalent uranium (U⁴⁺) to the more soluble hexavalent uranyl ion (UO₂²⁺) under oxidizing conditions, which then reacts with phosphate from surrounding rocks or fluids to precipitate as the hydrated calcium uranyl phosphate.^[11] The oxidation typically occurs in near-surface environments where oxygen-rich groundwater or meteoric waters interact with primary deposits, leaching and redepositing uranium as secondary phases like autunite.^[12] Autunite precipitation is favored in low-temperature hydrothermal veins, granite pegmatites, and supergene enrichment zones within oxidized uranium deposits.^[13] These settings involve circulating fluids at ambient to mildly elevated temperatures (often below 100°C), where the solubility of uranyl phosphate is controlled by specific pH and redox conditions.^[10] In oxidizing environments (high Eh), uranyl ions remain mobile, but precipitation occurs when pH ranges from 3 to 9, as solubility peaks below pH 3.5 or above pH 9; neutral to slightly acidic conditions in the 4–7 range promote the formation of stable autunite crystals from uranium-bearing solutions.^[11] Supergene processes, driven by weathering in arid or semi-arid regions, further concentrate autunite in fractures, breccias, or altered wallrocks of veins.^[14] The mineral commonly associates with other secondary uranium phosphates, including torbernite (Cu(UO₂)₂(PO₄)₂·10–12H₂O) and meta-autunite (the dehydrated form of autunite), as well as gypsum (CaSO₄·2H₂O) in phosphate-rich, evaporative settings.^[13] These associations arise during the precipitation from oxidizing, uranium-laden solutions that infiltrate permeable host rocks, such as quartzites, schists, or granites, often alongside iron oxides like goethite.^[12] The co-occurrence reflects shared geochemical pathways where phosphate availability and calcium concentrations dictate the paragenesis.^[14] In rare cases, autunite undergoes biogenic precipitation mediated by metal-resistant bacteria, such as Bacillus sp. and Rahnella sp., in uranium-contaminated soils.^[15] These microorganisms liberate phosphate through phosphatase activity on organic substrates, promoting the rapid immobilization of soluble U(VI) as autunite-like minerals under aerobic, acidic conditions (pH ~4–5), thereby reducing uranium mobility and aiding natural remediation in subsurface environments.^[15] This process has been observed to precipitate up to 95% of soluble uranium in contaminated soils at sites like the Oak Ridge Field Research Center.^[15]

Primary Locations

Autunite, a secondary uranium mineral, is rare but occurs widely in uranium-bearing provinces around the world, typically in the oxidized zones of hydrothermal veins, granite pegmatites, and altered primary uranium deposits.^[8] The type locality for autunite is Saint-Symphorien-de-Marmagne, near Autun in Saône-et-Loire, France, where it was first discovered in 1852 and named in 1854 from granite pegmatites associated with the alteration of uraninite.^[8] This site remains significant for its historical and mineralogical value, with autunite forming bright yellow, tabular crystals in the weathering products of uranium-rich pegmatites.^[8] In the United States, notable occurrences include the Daybreak Mine near Mount Spokane in Spokane County, Washington, where autunite is found in shallow pegmatite veins within granitic rocks, often as fluorescent, bladed crystals up to several millimeters across.^[8]^[16] Another key site is the Ruggles Mine in Grafton, New Hampshire, featuring autunite and meta-autunite in complex pegmatites, where it lines vugs and fractures alongside quartz and other phosphates. Europe hosts several important deposits, such as the Assunção Mine in Aldeia Nova, Ferreira de Aves, Sátão, Viseu District, Portugal, a uranium vein system in granitic terrain yielding tabular autunite crystals on quartz matrices.^[17] In the United Kingdom, autunite appears in Cornwall's metalliferous mining district, particularly at sites like South Tolcarne Mine near Camborne and Wheal Edward in St Just, associated with oxidized uranium veins in granite-hosted lodes.^[18]^[19] In South America, autunite occurs at sites like Malacacheta in Minas Gerais, Brazil. In Australia, notable occurrences include the Mt. Painter Mine in the Flinders Ranges, South Australia.^[8] While classic localities dominate documented occurrences, no major new discoveries of autunite post-2020 have been reported in scientific literature, though ongoing collections continue at established sites like Ruggles Mine.^[8]

Mining and Extraction

Autunite deposits are extracted primarily through open-pit mining for near-surface occurrences or underground methods for deeper vein and pegmatite-hosted ores, with the choice depending on deposit geometry and economics.^[20] Due to the mineral's fragility and its susceptibility to dehydration and flaking during handling, hand-sorting is commonly used to separate high-grade autunite from waste rock, minimizing mechanical damage and preserving ore quality.^[21] In the early 20th century, autunite mining in the United States focused on radium and uranium recovery, with initial operations in Washington state targeting secondary uranium minerals in granitic terrains.^[22] A significant post-World War II production boom, driven by nuclear demands, expanded these efforts; for instance, the Daybreak Mine near Spokane, Washington, yielded 13,400 tons of ore averaging 0.24% U₃O₈ through open-pit and underground workings by May 1958.^[23] Similarly, the Midnite Mine in Stevens County, Washington, operated as an open-pit from 1956 to 1962 and 1969 to 1982, producing more than 10 million pounds (approximately 13 million based on ore tonnage) of U₃O₈ from autunite-rich breccias along a granite-metasediment contact.^[24] Following extraction, autunite ore undergoes concentration via froth flotation to separate uranium-bearing phosphates from gangue or direct leaching to dissolve the mineral.^[25] Uranium recovery typically involves sulfuric acid dissolution under atmospheric or pressure conditions, often with oxidants like sodium chlorate to convert tetravalent uranium to soluble uranyl ions, achieving extraction efficiencies of 96-99.5%.^[25] Contemporary autunite mining remains limited, as most viable deposits are low-grade relative to modern economic thresholds, and operations face rigorous environmental regulations on radioactive tailings and waste management to prevent groundwater contamination and radon release.^[26]

History and Discovery

Etymology

Autunite derives its name from the city of Autun in Saône-et-Loire, France, which served as the type locality for the mineral near the village of Saint-Symphorien-de-Marmagne.^[4] The name reflects the 19th-century mineralogical convention of toponymic naming, where species were often designated after significant discovery sites to honor their geological origins. Prior to formal naming, it was known by the archaic term "calco-uranite," highlighting its calcium and uranium content.^[8] The mineral was formally named and described as a distinct species by British mineralogists Henry James Brooke and William Hallowes Miller in their 1852 edition of An Elementary Introduction to Mineralogy.^[4] This publication established autunite's identity as a hydrated calcium uranyl phosphate. The related term "meta-autunite" refers to the dehydrated variant of autunite, with the prefix "meta-" indicating a metamorphic or altered state due to loss of water molecules, as coined by French mineralogist Paul Gaubert in 1904.^[27] No other direct etymological derivations exist for autunite beyond its locational root.

Initial Discovery

Autunite was first identified in 1800 at the Saint-Symphorien locality near Autun, Saône-et-Loire, France, occurring as a secondary mineral in a pegmatite vein associated with the oxidation of primary uranium-bearing minerals.^[4] Specimens from this site were first chemically analyzed in 1824 by Jöns Jacob Berzelius, who described it as a calcium-based uranium phosphate compound.^[4] The mineral received its formal name, autunite, in 1852 from British mineralogists Henry James Brooke and William Hallowes Miller, based on detailed examination of material from the type locality; this initial scientific description distinguished it as a distinct species within the emerging catalog of uranium minerals.^[4]^[8] Early studies often confused autunite with the chemically similar torbernite due to their shared sheet-like structure and yellow-green coloration, but analyses confirmed autunite's unique calcium content versus torbernite's copper, establishing it as a separate hydrated uranyl phosphate.^[4] By 1850, autunite was one of approximately six known uranium minerals, including uraninite and torbernite, and its recognition as a secondary oxidation product played a key role in advancing 19th-century understanding of uranium geochemistry and mineral formation processes long before the discovery of radioactivity in 1896.^[4] This early documentation highlighted the role of secondary phosphates in uranium dispersal within oxidized environments, influencing subsequent classifications of hydrothermal and supergene mineral assemblages.^[4]

Varieties

Meta-autunite

Meta-autunite is the primary dehydrated variety of autunite, characterized by the chemical formula Ca(UO₂)₂(PO₄)₂·2–6H₂O and belonging to the tetragonal crystal system.^[28] It forms part of the meta-autunite group, representing a secondary mineral that results from the loss of water molecules from the more hydrated autunite structure.^[27] This dehydration leads to a more compact arrangement of uranyl phosphate sheets, distinguishing it within the series of uranyl phosphate minerals.^[28] In terms of properties, meta-autunite retains a similar appearance to autunite, typically occurring as lemon-yellow to greenish-yellow tabular crystals or foliated aggregates with a pearly to dull luster, but it is notably more stable under ambient conditions due to its reduced hydration.^[28] Its Mohs hardness ranges from 2 to 2.5, and while it fluoresces yellowish-green under ultraviolet light, the intensity is diminished compared to autunite.^[29]^[30] This stability makes meta-autunite less prone to further alteration in dry environments, though it remains radioactive owing to its uranium content.^[27] Meta-autunite commonly forms in situ through the dehydration of autunite, particularly in arid geological settings or during prolonged storage in low-humidity conditions where water loss occurs gradually.^[28] It is widespread in oxidation zones of uranium deposits, granite pegmatites, and sedimentary uranium-vanadium deposits, often appearing as pseudomorphs that preserve the original crystal shape of autunite.^[27] Distinction from autunite relies on changes observed in X-ray diffraction patterns, which reflect alterations in basal spacing and overall structure due to dehydration, such as key d-spacings around 8.46 Å for meta-autunite.^[28] Rather than being recognized as a fully separate mineral species, meta-autunite is classified as an end-member of the meta-autunite series, emphasizing its close structural relationship to the hydrated parent mineral.^[27]

Other Forms

Sodium-autunite is a rare sodium end-member in the autunite group, with the chemical formula Na₂(UO₂)₂(PO₄)₂·8H₂O, occurring as yellow crystals or coatings in oxidized uranium deposits.^[31] It forms through cation exchange in sodium-rich environments, often alongside meta-autunite and uranophane, and is noted for its potential to alter hydration states under varying conditions.^[31] Autunite can form pseudomorphs after primary uranium minerals such as uraninite, typically in secondary alteration zones of uranium-bearing pegmatites or veins.^[8] These pseudomorphs appear as tabular aggregates, resulting from the oxidation and replacement processes in hydrothermal settings.^[8] The autunite group includes other cation-substituted minerals structurally similar to autunite. Saléeite, the magnesium end-member with formula Mg(UO₂)₂(PO₄)₂·10H₂O, forms pale yellow coatings in magnesium-enriched oxidized zones; it shares paragenetic associations with autunite.^[32] These related minerals are uncommon and typically site-specific, arising from local impurities in uranium deposits.^[32]

Applications and Hazards

Uses in Industry

Autunite serves primarily as a uranium ore in the nuclear industry, where its high uranium content—approximately 48% by weight—makes it suitable for extraction processes that produce nuclear fuel. The mineral is milled and subjected to chemical leaching, typically with sulfuric acid, to dissolve the uranium, followed by purification steps such as solvent extraction and precipitation to yield yellowcake, a concentrate of uranium oxide (U₃O₈) that is further refined into fuel for nuclear reactors.^[8]^[2]^[33] In the early 20th century, autunite was a significant source for radium production, particularly from deposits in France and Portugal, where it was processed at facilities like the one at Nogent-sur-Marne to extract radium for applications including luminous paints used in medical and consumer products.^[34] Today, autunite plays a minor role in global uranium supply, as primary ores like uraninite dominate commercial production, but it remains relevant in geochemical prospecting for uranium deposits due to its distinctive yellow-green fluorescence under ultraviolet light, which aids in identifying potential sites during field surveys.^[35]^[36] Historically, before the full recognition of its radioactivity, autunite was employed as a pigment in ceramics, valued for its ability to produce reddish-yellow glazes when fired, though this use has been discontinued due to health concerns.^[2]

Radioactivity and Safety

Autunite is radioactive primarily due to its uranium content, which constitutes approximately 48.27% of its molecular weight in the formula Ca[(UO₂)₂(PO₄)₂]·(10–12)H₂O.^[5] The mineral emits alpha particles as part of the uranium-238 decay chain, where uranium-238 decays through a series of alpha and beta emissions to stable lead-206.^[37] Given natural uranium's specific activity of about 25,000 Bq/g, autunite's overall activity is roughly 12,000 Bq/g, reflecting its high uranium proportion and the contributions from decay chain daughters in secular equilibrium.^[38] Health risks from autunite arise mainly from internal exposure pathways rather than external contact. Inhalation of fine dust particles can lead to uranium accumulation in the lungs, causing inflammation, fibrosis, and an increased risk of lung cancer over time due to alpha particle irradiation of lung tissue.^[39] External exposure poses low immediate risk because alpha particles have limited penetration depth and do not reach living cells through intact skin, though prolonged contact may result in cumulative beta or gamma exposure from decay products.^[39] Ingestion, if dust contaminates food or hands, can cause kidney damage from chemical toxicity of soluble uranium species, exacerbating radiological effects.^[39] Safe handling of autunite requires adherence to established protocols to minimize exposure. Specimens should be stored in sealed, airtight containers to prevent dust release and radon gas accumulation, with ventilation systems in laboratories ensuring air exchange rates that keep airborne particulates below occupational limits.^[40] Direct skin contact should be avoided using gloves, and ingestion risks mitigated by prohibiting eating, drinking, or smoking in handling areas.^[41] Regulatory guidelines, such as those from the U.S. Nuclear Regulatory Commission, limit annual radiation dose to 50 mSv for radiation workers and 1 mSv for the public, with specific derived air concentration limits for uranium at 0.2 mg/m³ for soluble compounds.^[42] Environmentally, autunite in mining tailings contributes to long-term uranium contamination of soil and groundwater through leaching, where mobilized uranyl ions (UO₂²⁺) can spread via surface runoff or percolation.^[43] Remediation strategies often employ sorption techniques, using natural or engineered materials like iron oxides or biochar to adsorb uranium and reduce its mobility, thereby stabilizing sites and preventing bioaccumulation in ecosystems.^[44]

References

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Apr 1, 2012 · ... tetragonal symmetry, space group P4/ncc, with the unit-cell parameters: a = 6.9935(7), c = 17.5101(12) Å, V = 856.40(13) Å3, and Z = 4. The ...
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Autunite from. South Tolcarne Mine, Beacon, Camborne, Cornwall, England, UK ; Locality type: Mine ; Species: Autunite ; Formula: Ca(UO 2) 2(PO 4) 2 · 10-12H 2O.
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[43]
[PDF] Improving Mitigation of the Long-Term Legacy of Mining Activities
These data indicate that uranium migration will be delayed by adsorption of U(VI) on mineral surfaces during rehabilitation, provided circum-neutral pH and ...